Starscream was originally a scientist, along with Jetfire(also known as Skyfire), and Jetfire was lost in the arctic. Millions of years later when he was uncovered by Thundercracker and Rumble, Starscream recognized him and convinced him to join the decepticons.
When he discovered humans, and refused to destroy them, he found that his long time friend Starscream had changed. Being of science and not destruction Jetfire realizes, that Megatron and the decepticons are evil beings. When Starscream turns on him and tries to destroy him, Ratchet comes to his aid and repairs him. After this Jetfire joins the Autobots and their fight against Megatron and the Decepticons. Although the Autobots still distrust him to a point since he is compassionate and may hesitate during the fight.
During the fight with the decepticons, Jetfire sacrifices himself to save the autobots and earth, by imprisoning himself back into the ice in which he was found.